Large aircraft turbofan gas turbine engines have correspondingly large fan blades which may be about 1.5 meters long for example. In order to reduce the weight of the fan blades, they are typically formed as hollow, thin skinned members having internal stiffening ribs which typically define several radially extending passages therein.
Diffusion bonding is one conventional technique for forming the airfoils of the fan blades. In diffusion bonding, first and second airfoil preformed sections or halves are disposed together in abutting contact along a bond line or surface, with the leading and trailing edges of the airfoil being temporarily welded together so that the airfoil becomes pressure tight. The airfoil is then subjected to predetermined pressure-temperature-time conditions so that the airfoil sections may be metallurgically joined together at the bond line. Diffusion bonding does not rely on melting of the parent material as is required in conventional welding processes, but instead, the parent material undergoes plastic yielding and creeping at the elevated temperatures and pressures involved for forming an improved bond along the bond line.
In most cases, the equipment used to form diffusion bonds is custom built for the specific parts being bonded, with the bonding usually occurring in a vacuum or in a suitable inert gas. Diffusion bonding may occur at various elevated temperatures and pressures with varying degrees of plastic deformation of the components. For example, the skin of the airfoil may actually collapse around the internal ribs during the diffusion process and must be returned to a suitable configuration typically accomplished by internally pressurizing the airfoil to inflate the skin to its original form.
Diffusion bonding may occur at various pressures ranging from 1 to about 1,000 atmospheres, but it is desirable to conduct diffusion bonding at relatively low pressures of about 1 to 6 atmospheres. However, it is extremely difficult to diffusion bond at moderate pressures gas turbine engine fan blades formed of titanium with large surface areas. This is particularly true for leading and trailing edges of blade airfoils which are relatively rigid and spaced relatively far from the adjacent internal passages formed in the airfoil by the radially extending ribs therein. The diffusion bond line typically extends substantially equidistantly between the two airfoil halves from the leading edge to the trailing edge and can have a relatively large camber length from each of the leading and trailing edges to the next adjacent internal passage. It is difficult to provide pressure loading along the leading and trailing edges for ensuring a suitable amount of mating force along the bond line thereat for obtaining effective diffusion bonding thereof.
Furthermore, the mating surfaces of the airfoil halves along the leading and trailing edges, and specifically adjacent to the internal passage, may have irregularities therein which prevent the desired intimate contact therebetween. Such irregularities may result in incomplete bonding during the diffusion process, which may lead to rejection of the airfoil.
In one conventional arrangement, an airfoil has a bonding line extending generally along the camber centerline thereof between the leading and trailing edges, and the leading and trailing edges each includes respective extensions thereof having a mating bond line therebetween. The extensions are welded along the edge of the bond line for making the airfoil pressure tight so that external pressure may be applied for diffusion bonding together the airfoil halves. The extensions are then conventionally machined away revealing the final, finish contour of the leading and trailing edges. However, the extensions merely form extensions of the bond line along the airfoil camber line and therefore increase the difficulty of achieving a suitable diffusion bond.